The joint investigation team has reportedly found screw pieces of torpedo, probably causing the sinking of the Navy patrol ship Cheonan, near the border waters in the West Sea where the incident took place in late March.

The Korea Broadcast System (KBS) reported Tuesday that the team has launched close checking of the findings in a non-destructive testing. "The manufacturers of the screw are shortlisted to two countries Russia and China," KBS reported, quoting government officials who were not identified.

The screw, which is a part that creates power to propel the torpedo in the tail, has been regarded as a decisive clue to the cause of the incident as it does not destruct in general even during an explosion.

The government is ready to make it clear that the corvette was sunk by the torpedo and North Korea will be responsible for torpedo attack as the government has already secured pieces of material evidence, including screw part, when it announces investigation results May 20.